Bland, PA

Cressey, G

Genge, MJ

Grady, MM

Hutchison, R

Russell, SS

Zolensky, ME

English

Spiral - Imperial College Digital Repository

THE DISTRIBUTION OF METEORITE FINDS IN THE NAMIBIAN DESERT AND RECOVERY OF A HIGHLY SHOCKED METEORITE PAIRING GROUP.  S. S. Russell1, M. E. Zolensky2, P. A. Bland1, M. J. Genge1, G. Cressey1, M. M. Grady1 and R. Hutchison1.  1Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK (sarr@nhm.ac.uk); 2Johnson Space Center, Mail Code SN-2, NASA Road 1, Houston, TX USA.   Introduction: Meteorite searches of many hot desert areas have proved highly successful (e.g. for the Australian Nullarbor [1] and the Sahara desert [2]).  The Namibian desert is a prime candidate for organised meteorite searches.  It has extremely low rainfall (~2 inches a year) and has probably remained arid for at least the last 130 kyr.  A systematic search in 1991 in several locations in the region resulted in the recovery of 3 unpaired meteorites, Rooikop 001 (H5), Rooikop 002 (L5), Rooikop 003 (L4/5) [3].  Meteorite Search:  In July 1999, four of us (PAB, MJG,  SSR, and MEZ)  spent two weeks searching for meteorites in the Rooikop area, about 20km East of Walvis Bay.  This area was selected primarily because of the success of Reid et al. (1995) [3] in recovering meteorites from this region.  It is a flat-lying area, composed of late Cenozoic sediments, with many ancient deflation surfaces [4]. Permission to search was obtained by the Namibian Geological Survey, and the meteorites recovered during the expedition remain the property of Namibia, but are currently on loan to NHM. Specific search areas were located using satellite images and topologic maps, and when in the field the amount of local vegetation and degree of soil compaction was also a consideration.   Because of the typically rust-coloured to dark brown host rock, it was necessary to search the area by foot.  The location of each meteorite found was recorded by GPS.     Fig 1. Met 007 in the field. (Note: the hammer is ~10cm long)  Results: Thirteen meteorites were recovered (e.g. Fig. 1), all of which were either single stones or made up of several pieces that clearly fit together.  Most of the meteorites were located on the sides of slopes or on plateaux, and lower lying areas proved less successful to search, perhaps because these areas experienced infrequent fluvial activity that promoted the weathering of any meteorites. The total amount of extraterrestrial material recovered was 1248g.  The meteorites are moderately weathered and most are shock stage S3 (contain maskelinite); two (Mets 009 and 010) are shock stage S6 (see below).  All of the specimens are L chondrites, and all are fully equilibrated, of petrologic grade 6.  A summary of preliminary petrologic observations and data is given in Table 1.  Two meteorites, Met 009 and Met 010, are highly shocked. They contain around 30% blackened shock veins, a few mm in width.  Inspection of these veins by optical microscopy showed that they are rich in the rare, ultra-high pressure minerals ringwoodite and majorite, the high pressure polymorphs of olivine and pyroxene respectively. The presence of these minerals was confirmed by XRD; Fig 2. The chemical composition of most samples of ringwoodite is approximately the same as that of the olivine in the host meteorite (e.g. FeO ~23.5%) affirming that ringwoodite formed from the meteoritic olivine, presumably during a shock event in the asteroid belt.  However, in patches the iron abundance of the ringwoodite is significantly higher than in the olivine, up ~29%FeO, equivalent to Fa33, perhaps because ringwoodite is prone to exchange with terrestrial iron during weathering under hot desert conditions. Discussion: The pairing of the meteorites found has not yet been fully established; however the variation in shock stage between samples suggests that the meteorites are not all from a single pairing group.  Furthermore, none of the meteorites we found are paired to the already-known Rooikop samples and so our samples represent new extraterrestrial material.  Once the pairing characteristics of the specimens has been ascertained, a report will be submitted to the Nomenclature committee suggesting the meteorites be named Rooikop 004-00X.  These finds confirm that the Namibian desert is a fertile search area for meteorites.  References:   [1] Bevan A. and Binns R. A. (1989) Meteoritics 24 127-133; [2] Bischoff A. and Geiger T. (1995) Meteoritics 30 113-122; [3] Reid A. et al., (1995) Meteoritics 30 781-784; [4] Wilkinson M. J. (1990) Paleoenvironments in the Namib desert: The Lower Tumas Basin in the late Cenozoic. Univ. Chicago Geog. Res. Paper 231 196pp.  Lunar and Planetary Science XXXI 1743.pdfSHORT TITLE HERE:  A. B. Author and C. D. Author  Acknowledgements:  We are grateful to Dr Gabi Schneider, Director, Geological Survey of Namibia, for her advice and permission to undertake the meteorite collection. We would like to thank Justin Wilkinson for advice in selecting meteorite search areas. The expedition was funded by an Enhancement Grant from the Natural History Museum.  Table 1:  Preliminary observations of meteorites found near Roikoop, July 1999 Field Name Weight (g) Shock stage Fa Fs Classification Comments Met 006 61.4 S3 25 22 L6  Met 007 158.1 S3 26 24 L6  Met 008 168.5 S3 26 21 L6  Met 009 135.5 S6 25 22 L6 Contains ringwoodite +majorite Met 010 79 S6 28 21 L6 Contains ringwoodite +majorite Met 011 48.8 S3 25 22 L6  Met 012 24.1 S3 25 22 L6  Met 013 125 S3 25 22 L6  Met 014 25 S3 25 23 L6  Met 015 202.8 S3 26 22 L6  Met 016 55.7 S3 26 22 L6  Met 017 99.7 S3 25 22 L6  Met 018 64.7 S3 25 22 L6   30 35 40 45 500100200300Namibia Met 009 (L6 shocked)Scanning diffractometer.Polished section sample.Step 0.01° 2 thetaCount time per step = 8 secM + ROMOOROOO M + R2θ Cu KαCounts Fig 2. Bulk XRD pattern taken for Met 009 using a polished thin section.  M=majorite peak; R= ringwoodite peak; O= olivine peak. Lunar and Planetary Science XXXI 1743.pdf

The distribution of meteorite finds in the Namibian desert and recovery of a highly shocked meteorite pairing group.

http://spiral.imperial.ac.uk/bitstream/10044/1/1051/1/The%20distribution%20of%20meteorite%20finds.pdf

The distribution of meteorite finds in the Namibian desert and recovery of a highly shocked meteorite pairing group.

Abstract

Similar works

Full text

Available Versions

Spiral - Imperial College Digital Repository